In the papermaking industry certain applications require edge trimming of the paper web during web travel in the papermaking machine. One such application occurs in the manufacture of coated paper. Coating can be done "on-machine" or "off-machine". Coating on-machine means that the paper is made in the normal fashion and an additional coating station is provided such that the paper is coated as it travels in the papermaking process.
During the coating process the "raw stock" paper is coated across the width of the web except for the edges of the paper where a trim of 0.5 to 3 inches of the paper web is not coated. Both sides of the paper web may be coated with plastic or paint products which can make up a significant part of the paper weight. The uncoated edges must be trimmed off before processing the paper further, such processing involving calendering or supercalendering, for example. This trimming step is typically done by means of a mating pair of rotary slitting knives. Slitting knives, or "slitters" are used to trim paper edges after the coating station thereby removing un-coated edges of the paper from the traveling web. Slitters are also employed upstream of the coating station to cut the paper web prior to coating preventing cracked edges from entering the coater because coating of cracked edges leads to further complications.
There are disadvantages associated with the use of knives. Knives require high maintenance due to abrasive wear of the coating filler material, i.e. calcium carbonate or titanium dioxide. Further, slitter knives have a tendency to separate (jamming) when flying splices of overlapping paper sections joined together by tape having triple the basis weight pass by the knife slitters. Jamming leads to paper breaks. Also, slitter knives are very sensitive to paper wrinkles that enter the slitting zone resulting in edge tearing of the paper. Further, it is very difficult to enter the running paper web into the paired slitter knives without crimping the edge which can lead to web breaking.
For the above stated reasons, it is standard practice in the paper making industry to avoid cutting the trim on the on-machine coater, because one paper break may cost up to $8000 in lost production time. To avoid the potential hazards associated with on-machine trimming, trimming is usually done off-machine on a re-reeler machine which is a separate machine used to un-reel, edge trim and re-reel the paper web. Such a re-reeler operation requires significant labour costs to operate in addition to the capital costs of purchasing such a machine. Clearly, on-machine trimming of the edge of the paper web during the coating section of the machine would be the preferred choice of manufacture if the risks associated with web breakage by edge trimming after the coating station could be avoided or reduced.
If a water jet could be used to trim the edge of the coated paper, then the disadvantages associated with the use of rotary knives in the on-machine edge trimming would be avoided. The pressure of the water jet is sufficient to cut through flying splices of triple basis weight on the coated paper. Further, changes in tension which can cause paper wrinkling have no apparent effect on the water jet cutting ability to provide a clean cut without tearing.
While it is known to use water jets to cut a paper web, the general teaching in the art is to cut the web with the water jet impinging the web surface in a direction normal to the web surface to maximize the cutting efficiency of the waterjet. However, when this teaching was applied by the applicant to coated paper webs, some disturbing problems were experienced which adversely affected the cutting performance of the water jet.
One problem experienced with using such a normal directed water jet to effect trimming of the coated paper is backsplash. It should be understood that the water jet pressure and nozzle orifice can be chosen such that the water jet impinges the paper web at speeds in excess of 2000 feet/second or about 615 meters/second. The energy of this impact transforms the fluid into a gaseous state. This represents expansion in volume in the order of 1250 times. Consequently the gas stream formed by the water jet impinging the web results in a cloud of fumes when the paper is of sufficient weight and thickness to backsplash minute portions of the water jet spray. The fumes and atomized paper particles backsplash towards the nozzle head of the water jet. The coating mass on the paper which contains calcium carbonate is splashed from the cutting surface back to the nozzle head and is also sprayed on other parts of the paper. During use, the fine particles of calcium lump together to form and build up extending down from the nozzle to the surface of the paper in a gummy "stalacite" type of deposit. Eventually this "stalactite" type of deposit breaks off in a lump and travels on the boundary layer of the good paper web to the wind up station where the lumped particles are glued to the next layer of paper. Upon un-winding, the glued together layers are ripped at the edge leading to complete web breaks across the paper web. Clearly, this will result in down time to re-splice and feed the tail of the paper web through the machine. Furthermore, splattering of the calcium deposits on the boundary edge of the coated paper deteriorates paper quality.
The use of water jets to perform slitting, tail cutting and trimming has been disclosed in the patent literature, but there is no disclosure as to how to use the water jets to cut an "unfriendly" paper web such as coated paper during on-machine operation. For example, U.S. Pat. No. 3,996,825 issued Dec. 14, 1976 to Rupert Terry discloses a water jet cutter for cutting and trimming a web. A first water jet impinges the web from below the web at a normal angle of 90.degree. to the plane of the web. A second water jet impinges the web from above the web at the same point as the lower water jet impinges the web. The upper or second water jet is angled relative to the vertical axis or axis normal to the plane of the web to direct the second water jet towards the outside edge of the paper web such that the water jet forces the cut web edge away from the existing web to the outside of the cutting station. The patent teaches that various angles relative to the vertical can be used by the second water jet as long as the jet is emitted directly downwardly and toward the outside edge. A suitable angle is typically about 45 degrees. Such a trimmer would not work in cutting coated paper because splashing from each water jet onto the other water jet is subject to the problems associated with the above noted trimming of coated paper on-machine.
U.S. Pat. No. 5,068,513 issued Nov. 26, 1991 to Donald Gangemi teaches a water jet cutter that is used with a laser as a slitter. In one embodiment, the water jet cutter is angled rearwardly or upstream with respect to the direction of web travel. However, the patent teaches that the relative orientation of the water jet nozzle and the laser is not deemed to be of critical importance so long as the water jet nozzle and the laser are focused at the same general spot on the traveling web. Also, DE patent application 4,28,272 laid open Oct. 29, 1992 in the name of J. M. Voith GmbH discloses the use of either rotary cutters, water jets or lasers to cut a tail in a paper by angling the resultant cut with a vector component in opposing direction to the direction of web travel and in a second direction into the center of the web away from the edge of the web. Again these patents are not concerned with backsplash and would be subject to waste deposit build upon the nozzle heads and laser heads.
While prior art water jet cutters for use in tail cutting, slitting and to a lesser extent trimming are known in the paper making industry, none of these patents addresses the problems associated with backsplash and the effects of backsplash on clotting the nozzle and effecting paper quality as a result of cutting through coated paper on-machine. There is a need for a water jet edge trimming system which is not subject to the disadvantages associated with the knife edge trimmers of the prior art.